I'm also speaking off-line with a bench-rest shooter, who also builds AR-15 rifles, and he seems to think that the additional hardness helps with the rate at which the muzzle "bells out" as higher round counts/rates of round counts increases. This is more prominent with threaded barrels than unthreaded barrels, btw.

That appears to make sense on the surface.

I know from working on aviation steels, that it is much tougher to screw up the lower numbered steels. 4130 is the easiest to work on, while 4140 and 4150 become harder to work without making them overly brittle or wearing out your tools. In other words, if you don't have perfect quality control, 4140 has a better chance of turning out "tougher" than 4150. The additional hardness potential of 4150 requires a higher QC, in other words.

I seem to recall something about heat dissipation superiority of 4150 over 4140.